The client you are using to log in to the game is out of date or there is a mismatch between the client and server versions. This error can occur if there was a recent update to the game and you haven't patched. Follow these steps to resolve the issue:
- Use the Blizzard Battle.net desktop app to launch the game. Any pending game updates will download once the application is opened.
Run the repair tool to repair any damaged game files.